The present invention relates to a charging device in an image forming apparatus such as an electrophotographic apparatus, an electrostatic recording apparatus, or the like, in an electrophotographic copying machine, a printer, a facsimile, composite OA appliances thereof, etc. More particularly, the present invention relates to a charging device which is pressed against a surface of a body to be charged such as a light-sensitive body, a dielectric body, or the like, to thereby charge the surface of the body to be charged uniformly.
In an image forming apparatus such as an electrophotographic apparatus, an electrostatic recording apparatus, or the like, such an operation is to charge a surface of a body to be charged such as a light-sensitive body, a dielectric body, or the like. As a charging means, there is generally used a contactless charging system in which charging is performed by corona discharge which is generated by application of a high voltage to a tungsten wire. In such a contactless charging system, however, a large amount of ozone or nitrogen oxide (NO.sub.x) is generated to bring about environmental pollution in the periphery of the image forming apparatus. Further, there arises a problem that the surface of the light-sensitive body is denatured by a corona product to cause deterioration of the light-sensitive body or image fogging and that pollution of wire has an influence on image quality to cause white spotting or black streaking in image.
On the contrary to the aforementioned contactless charging system, there is a contact charging system in which charging is performed by bringing a charging device into contact with a body to be charged. The contact charging system has an advantage that the voltage to be applied to the charging device is low so that the quantity of generated ozone becomes very low.
Also the contact type charging device, however, has a lot of problems, and various proposals have been made. For example, there is known an electrically conductive roll which has a base layer formed from an electronic electric conduction type elastic body material having a specific volume resistance adjusted to a value not larger than 10.sup.3 .OMEGA.cm, and an electrode layer formed from a synthetic resin material adjusted to a value of about 10.sup.2 .OMEGA.cm. Because the specific volume resistance of the base layer and the specific volume resistance of the electrode layer are both small, abnormal electric discharge is caused easily by uneven dispersion of the electrically conductive agent. Further, because the resistance against leaking depends on a dielectric layer formed on the electrode layer and having a film thickness of about 160 .mu.m, the electrically conductive roll has a problem that leaking occurs easily.
As a measure, the Unexamined Japanese Patent Application Publication No. Hei 7-77859 has proposed an electrically conductive roll in which a base layer formed from an ionic electric conduction type elastic body having a specific volume resistance of from 10.sup.6 .OMEGA.cm to 10.sup.9 .OMEGA.cm, an electrode layer formed from an electronic electric conduction type synthetic resin having a specific volume resistance not larger than 10.sup.3 .OMEGA.cm, a resistance regulation layer and a protective layer are provided successively on the outer circumferential surface of a shaft body. Further, the Unexamined Japanese Patent Application Publication No. Hei 7-28301 has proposed an electrically conductive roll in which the aforementioned base layer formed from a foamed elastic body, an electronic electric conduction type auxiliary layer having a specific volume resistance not larger than 10.sup.4 .OMEGA.cm and a thin protective layer are provided successively on the outer circumferential surface of a shaft body. In these electrically conductive rolls, it is said that the abnormal discharge voltage is improved effectively to prevent the occurrence of leaking because the dispersion of the voltage distribution is attained at the time of application of the voltage by the base layer of medium resistance.
In the electrically conductive rolls disclosed in the aforementioned publications, the base layer is, however, formed from an elastic body material containing a small amount of ionic electric conduction agent, so that charging characteristic varies widely correspondingly to the environmental change because the electrically conductive roll highly depends on environment. Further, because the electrode layer is formed by mixing an electronic electric conduction agent in a synthetic resin in the same manner as in the conventional electrically conductive roll, local uneven dispersion of the electric conduction agent cannot be avoided. Accordingly, there arises a problem that not only leaking is still apt to be caused by the generation of abnormal electric discharge but also density irregularity occurs in a halftone image portion.
Further, in any one of the conventional contact type charging devices, toner and its outer additive, paper dust, etc. are deposited on a surface of the charging device through a body to be charged (light-sensitive body) when the charging device is used for a long time. There arises also a problem that the contact surface of the charging device is contaminated gradually by the deposition of toner, etc. so that the resistance value of the charging device is increased partially to thereby bring about lowering of charging characteristic.
As a measure, for example, the Unexamined Japanese Patent Application Publication No. Hei 4-303861 has proposed a charging device in which a stratiform solid lubricant such as graphite, or the like, is contained in a surface layer to make toner, etc. be hardly deposited on the surface of the charging device. In the charging device, the solid lubricant having a large particle size of 100 .mu.m is, however, present in the surface layer. Accordingly, there arises a problem that unevenness of resistance is brought -so that image quality failures caused by bias leakage and image density irregularity occur.